Haemophilus ducreyi, the etiologic agent of chancroid, is a major cause of genital ulcers worldwide and is increasing in prevalence in the United States. Infection with H. ducreyi appears to increase the transmission of the human immunodeficiency virus in populations where both diseases are endemic. There is no information identifying the surface components of the organism that allow it to invade epithelial surfaces and cause ulcers. The long term goal of this project is to examine the hypothesis that the major outer membrane protein (OMP) play a role in the pathogenesis of infections due to H. ducreyi. The immediate goal is to examine whether the major OMP has potential as an immune target and to examine several strains for conserved epitopes on this protein. To begin to elucidate the structure and function of the major OMP, the specific aims of this application include: 1) purification and analysis of the major OMP from a recent H. ducreyi isolate; 2) development of immunologic reagents specific for the major OMP; 3) cloning and sequencing the gene encoding the major OMP; 4) localization of epitopes on the major OMP; 5) characterization of the human immune response to the major OMP; 6) evaluation of a mouse model of H. ducreyi infection and examining whether antibodies directed against the major OMP prevent infection in this model.